1. Field of the Invention
The present invention relates generally to metering devices for electric current and more particularly, to current metering devices utilizing fiber optic techniques to measure the amount of current passing through a conductor.
2. Background Art
The possibility of sensing magnetic fields by the use of optical fibers has long been recognized, and several types of fiber optic magnetic sensors are known. The most successful and sensitive such magnetic sensing devices utilize the magnetostrictive properties of metal alloys. Areas of use for very sensitive magnetic field sensors has been in the field of magnetometer design to measure magnetic fields, and transformers, where a reliable and sensitive measurement of magnetic field strength provides an accurate indicator of the amount of electric current being conducted by a conductor. The use of magnetostriction and its properties have been well documented.
Magnetostriction is defined as the change in dimension of a ferromagnetic material when it is subjected to a magnetic field. The magnetostriction may be positive or negative depending on the material and its composition. For example, pure nickel displays negative magnetostriction, whereas iron-nickel alloys, having less than forty-five percent nickel, display positive mangetostriction. Moreover, data for 42-permalloy, which is an alloy of forty-two percent nickel and fifty-eight percent iron, has been shown to be about 3.5 times as sensitive as nickel alone. A study of the magnetostrictive properties of various materials may be found in an article in Applied Optics. Volume 19, No. 22 to Jarzynski et al, one of the authors being C. M. Davis, Jr., one of the inventors herein, which was published Nov. 15, 1980, and entitled "Magnetic Field Sensitivity of an Optical Fiber with Magnetostrictive Jacket".
The broad concept of impressing a phase modulation onto a fiber optic cable is also known in the art. Modulating means, such as a magnetostrictive element, may be coupled to the fiber without breaking the fiber. In fiber optic magnetic sensors, a single mode optical fiber is bonded to a magnetostricitive element or sheathed by a magnetostrictive jacket, either of which may undergo a longitudinal strain when a magnetic field is applied. This strain is transmitted to the optical fiber and effects the phase delay of a laser light beam propagating through the fiber. Phase changes in the optical fiber created by the strain can be detected using currently available technologies, and the strength of the magnetic field can thus be determined. Devices have been developed which can measure either very strong magnetic fields or alternatively very weak magnetic field on the order of 10.sup.-5 and 10.sup.-8 gauss.
In the field of high intensity electric current measurement, various means have been used in the past to measure electric current passing through a conductor. Conventional means such as Hall effect gauges, shunt type current measuring devices, etc., are difficult and sometimes dangerous to use, and are liable to vary considerably sometimes up to ten percent, from the actual current being conducted. Other devices, such as the one disclosed by U.S. Pat. No. 4,542,338, to Arditty et al, require continual electrical attachment to the conductor in which the current is being measured. Moreover, even the Arditty et al device fails to measure the electric current conducted in the intensity ranges presently utilized.
Other devices, such as U.S. Pat. Nos. 4,348,587 and 4,622,460 can detect magnetic fields of very small magnitudes, but are not usable to detect magnetic fields created by high intensity current conductors such as electric power cables. The difficulty heretofore has been the provision of a relatively simple, inexpensive yet highly accurate magnetostrictive sensor described which is capable of sensing magnetic fields created by a conductor carrying current in a range, for example, of from five to ten thousand amperes with an accuracy exceeding 0.1 percent.
A further drawback to the measuring transformer devices presently known concerns the installation of the devices which are difficult and sometimes dangerous to install and dismantle. This has led to devices which are permanently affixed to the conductors in which the current is being measured.
Magnetostrictive materials, like all ferromagnetic materials, exhibit hysteresis to some extent. Hysteresis is the residual material magnetization of a ferromagnetic material which remains upon the removal of a magnetic field and which only dissipates with time. As a result, hysteresis can influence the accuracy of a magnetostrictive measuring device. What is required is a device which can compensate for the effects of hysteresis and produce accurate measurements.